The auditory system is generally comprised of an external ear, a middle ear and an internal ear. The external ear includes the auricle (i.e., the ear flap) and auditory canal, while the internal ear includes the oval window and the vestibule which is a passageway to the cochlea. The middle ear is positioned between the external ear and the middle ear, and includes the eustachian tube, the tympanic membrane or eardrum, and three bones called ossicles, and the middle ear space. The three ossicles (i.e., the malleus, incus, and stapes), are positioned between and connected to the tympanic membrane and the oval window.
In a person with normal hearing, sound enters the external ear, where it is slightly amplified by the resonant characteristics of the auditory canal of the external ear. The sound waves produce vibrations in the tympanic membrane. The force of these vibrations is magnified by the ossicles.
Upon vibration of the ossicles, the oval window conducts the vibrations to cochlear fluid in the inner ear, thereby stimulating receptor cells or hairs within the cochlea. In response to the stimulation, the hairs generate an electrochemical signal that is delivered to the brain via one of the cranial nerves, allowing the brain to perceive sound.
A number of auditory system defects impair or prevent hearing. Some patients have ossicles that lack the resiliency necessary to increase the force of vibrations to a level that will adequately stimulate the receptor cells in the cochlea. Other patients have ossicles that are broken, and which therefore do not conduct sound vibrations to the oval window. However, in most cases, sensorineural hearing loss is due to the lack of proper hair cell function within the cochlea.
Prostheses for ossicular reconstruction are sometimes implanted in patients who have partially or completely broken ossicles. These prostheses are normally cut to fit snugly between the tympanic membrane and the oval window or stapes. The close fit holds the implants in place, although gelfoam is sometimes packed into the middle ear to ensure against loosening. Two basic forms are available: total ossicle replacement prostheses (TORPs) which are connected between the tympanic membrane and the oval window; and partial ossicle replacement prostheses (PORPs) which are positioned between the tympanic membrane and the stapes or between the incus and stapes or between the incus and oval window.
Although these prostheses provide a mechanism by which vibrations may be conducted through the middle ear to the oval window of the inner ear, additional devices are frequently necessary to ensure that vibrations are delivered to the inner ear with sufficient force to produce high quality sound perception. Even when a prosthesis is not used, disease and the like can result in hearing impairment.
Various types of hearing aids have been developed to restore or improve hearing for the hearing impaired. With conventional hearing aids, sound is detected by a microphone, amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or transducer into the middle ear by way of the tympanic membrane. Often the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion.
Attempts have been made to eliminate the feedback and distortion problems associated with conventional hearing aid systems. These attempts have yielded devices that convert sound waves into electromagnetic fields having the same frequencies as the sound waves. A microphone detects the sound waves, which are both amplified and converted to an electrical current. The current is delivered to a coil winding to generate an electromagnetic field which interacts with the magnetic field of a magnet positioned in the middle ear. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear or the skull.
Existing electromagnetic transducers present several problems. Many are installed using complex surgical procedures which present the usual risks associated with major surgery and which also require disarticulating (disconnecting) one or more of the bones of the middle ear. Disarticulation deprives the patient of any residual hearing he or she may have had prior to surgery, placing the patient in a worsened position if the implanted device is later found ineffective in improving the patient's hearing. Thus, the sound produced by these devices includes significant distortion because the vibrations conducted to the inner ear do not precisely correspond to the sound waves detected by the microphone.
In addition to the problems described above with most hearing aids presently in use, methods to assess the functioning of such devices when worn by users are lacking. For example, some methods (e.g., commercially available test systems) used to measure hearing aid performance require very expensive equipment, are expensive to implement, and difficult to use. In addition, these systems can provide misleading results when improperly used. What is needed in the art is an easy to use device for assessment of hearing devices, suitable for use prior to, during and after installation of such devices. In this manner, the proper functioning of the hearing device can be readily assessed and alternative treatment methods considered, should the need arise.